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                <time  class="post-full-meta-date" datetime="2017-10-22T16:00:00.000Z" itemprop="datePublished">
                    2017-10-23
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            <h1 class="post-full-title">使用 tc netem 模拟网络异常</h1>
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                    <p>在某些情况下，我们需要模拟网络很差的状态来测试软件能够正常工作，比如网络延迟、丢包、乱序、重复等。linux 系统强大的流量控制工具 tc 能很轻松地完成，tc 命令行是 <code>iproute2</code> 软件包中的软件，可以根据系统版本自行安装。</p>
<p>流量控制是个系统而复杂的话题，tc 能做的事情很多，除了本文介绍的还有带宽控制、优先级控制等等，这些功能是通过类似的<strong>模块</strong>组件实现的，这篇文章介绍的功能主要是通过 <code>netem</code> 这个组件实现的。<code>netem</code> 是 <code>Network Emulator</code> 的缩写，关于更多功能以及参数的详细解释可以参阅 <code>tc-netem</code> 的 man page。</p>
<h2 id="网络状况模拟"><a href="#网络状况模拟" class="headerlink" title="网络状况模拟"></a>网络状况模拟</h2><p>网络状况欠佳从用户角度来说就是下载东西慢（网页一直加载、视频卡顿、图片加载很久等），从网络报文角度来看却有很多情况：延迟（某个机器发送报文很慢）、丢包（发送的报文在网络中丢失需要一直重传）、乱序（报文顺序错乱，需要大量计算时间来重新排序）、重复（报文有大量重复，导致网络拥堵）、错误（接收到的报文有误只能丢弃重传）等。</p>
<p>对于这些情况，都可以用 netem 来模拟。需要注意的是，netem 是直接添加到网卡上的，也就是说所有从网卡发送出去的包都会收到配置参数的影响，所以最好搭建临时的虚拟机进行测试。</p>
<p>在下面的例子中 <code>add</code> 表示为网卡添加 netem 配置，<code>change</code> 表示修改已经存在的 netem 配置到新的值，如果要删除网卡上的配置可以使用 <code>del</code>：</p>
<pre><code># tc qdisc del dev eth0 root
</code></pre><h3 id="1-模拟延迟传输"><a href="#1-模拟延迟传输" class="headerlink" title="1. 模拟延迟传输"></a>1. 模拟延迟传输</h3><p>最简单的例子是所有的报文延迟 100ms 发送：</p>
<pre><code># tc qdisc add dev eth0 root netem delay 100ms
</code></pre><p>如果你想在一个局域网里模拟远距离传输的延迟可以用这个方法，比如实际用户会访问外国网站，延迟为 120ms，而你测试环境网络交互只需要 10ms，那么只要添加 110 ms 额外延迟就行。</p>
<p>在我本地的虚拟机中实验结果：</p>
<pre><code>[root@node02 ~]# tc qdisc replace dev enp0s8 root netem delay 100ms
[root@node02 ~]# ping 172.17.8.100
PING 172.17.8.100 (172.17.8.100) 56(84) bytes of data.
64 bytes from 172.17.8.100: icmp_seq=1 ttl=64 time=101 ms
64 bytes from 172.17.8.100: icmp_seq=2 ttl=64 time=100 ms
64 bytes from 172.17.8.100: icmp_seq=3 ttl=64 time=102 ms
^C
--- 172.17.8.100 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 100.725/101.370/102.048/0.653 ms
</code></pre><p>如果在网络中看到非常稳定的时延，很可能是某个地方加了定时器，因为网络线路很复杂，传输过程一定会有变化。因此实际情况网络延迟一定会有变化的，<code>netem</code> 也考虑到这一点，提供了额外的参数来控制延迟的时间分布，完整的参数列表为：</p>
<pre><code>DELAY := delay TIME [ JITTER [ CORRELATION ]]]
    [ distribution { uniform | normal | pareto |  paretonormal } ]
</code></pre><p>除了延迟时间 <code>TIME</code> 之外，还有三个可选参数：</p>
<ul>
<li><code>JITTER</code>：抖动，增加一个随机时间长度，让延迟时间出现在某个范围</li>
<li><code>CORRELATION</code>：相关，下一个报文延迟时间和上一个报文的相关系数</li>
<li><code>distribution</code>：分布，延迟的分布模式，可以选择的值有 <code>uniform</code>、<code>normal</code>、<code>pareto</code> 和 <code>paretonormal</code></li>
</ul>
<p>先说说 <code>JITTER</code>，如果设置为 <code>20ms</code>，那么报文延迟的时间在 100ms  ± 20ms 之间（90ms - 110ms），具体值随机选择：</p>
<pre><code>[root@node02 ~]# tc qdisc replace dev enp0s8 root netem delay 100ms 20ms
[root@node02 ~]# ping 172.17.8.100
PING 172.17.8.100 (172.17.8.100) 56(84) bytes of data.
64 bytes from 172.17.8.100: icmp_seq=1 ttl=64 time=112 ms
64 bytes from 172.17.8.100: icmp_seq=2 ttl=64 time=89.7 ms
64 bytes from 172.17.8.100: icmp_seq=3 ttl=64 time=114 ms
......
</code></pre><p><code>CORRELATION</code> 指相关性，因为网络状况是平滑变化的，短时间里相邻报文的延迟应该是近似的而不是完全随机的。这个值是个百分比，如果为 <code>100%</code>，就退化到固定延迟的情况；如果是 <code>0%</code> 则退化到随机延迟的情况</p>
<pre><code>[root@node02 ~]# tc qdisc replace dev enp0s8 root netem delay 100ms 20ms 50%
[root@node02 ~]# ping 172.17.8.100
PING 172.17.8.100 (172.17.8.100) 56(84) bytes of data.
64 bytes from 172.17.8.100: icmp_seq=1 ttl=64 time=116 ms
64 bytes from 172.17.8.100: icmp_seq=2 ttl=64 time=89.7 ms
64 bytes from 172.17.8.100: icmp_seq=3 ttl=64 time=90.8 ms
64 bytes from 172.17.8.100: icmp_seq=4 ttl=64 time=96.4 ms
64 bytes from 172.17.8.100: icmp_seq=5 ttl=64 time=90.5 ms
</code></pre><p>报文的分布和很多现实事件一样都满足某种统计规律，比如最常用的正态分布。因此为了更逼近现实情况，可以使用 <code>distribution</code> 参数来限制它的延迟分布模型。比如让报文延迟时间满足正态分布：</p>
<pre><code># tc qdisc change dev eth0 root netem delay 100ms 20ms distribution normal
[root@node02 ~]# tc qdisc replace dev enp0s8 root netem delay 100ms 20ms distribution normal
[root@node02 ~]# ping 172.17.8.100
PING 172.17.8.100 (172.17.8.100) 56(84) bytes of data.
64 bytes from 172.17.8.100: icmp_seq=1 ttl=64 time=119 ms
64 bytes from 172.17.8.100: icmp_seq=2 ttl=64 time=102 ms
64 bytes from 172.17.8.100: icmp_seq=3 ttl=64 time=115 ms
64 bytes from 172.17.8.100: icmp_seq=4 ttl=64 time=105 ms
64 bytes from 172.17.8.100: icmp_seq=5 ttl=64 time=119 ms

</code></pre><p>这样的话，大部分的延迟会在平均值的一定范围内，而很少接近出现最大值和最小值的延迟。</p>
<p>其他分布方法包括：<a href="https://en.wikipedia.org/wiki/Uniform_distribution_(continuous" target="_blank" rel="noopener">uniform</a>)、<a href="https://en.wikipedia.org/wiki/Pareto_distribution" target="_blank" rel="noopener">pareto</a> 和 <code>paretonormal</code>，这些分布我没有深入去看它们的意思，感兴趣的读者可以自行了解。</p>
<p>对于大多数情况，随机在某个时间范围里延迟就能满足需求的。</p>
<h3 id="2-模拟丢包率"><a href="#2-模拟丢包率" class="headerlink" title="2. 模拟丢包率"></a>2. 模拟丢包率</h3><p>另一个常见的网络异常是因为丢包，丢包会导致重传，从而增加网络链路的流量和延迟。netem 的 <code>loss</code> 参数可以模拟丢包率，比如发送的报文有 50% 的丢包率（为了容易用 ping 看出来，所以这个数字我选的很大，实际情况丢包率可能比这个小很多，比如 <code>0.5%</code>）：</p>
<pre><code>[root@node02 ~]# tc qdisc change dev enp0s8 root netem loss 50%
[root@node02 ~]# ping 172.17.8.100
PING 172.17.8.100 (172.17.8.100) 56(84) bytes of data.
64 bytes from 172.17.8.100: icmp_seq=1 ttl=64 time=0.716 ms
64 bytes from 172.17.8.100: icmp_seq=3 ttl=64 time=0.713 ms
64 bytes from 172.17.8.100: icmp_seq=5 ttl=64 time=0.719 ms
64 bytes from 172.17.8.100: icmp_seq=7 ttl=64 time=0.938 ms
64 bytes from 172.17.8.100: icmp_seq=10 ttl=64 time=0.594 ms
64 bytes from 172.17.8.100: icmp_seq=11 ttl=64 time=0.698 ms
64 bytes from 172.17.8.100: icmp_seq=12 ttl=64 time=0.681 ms
</code></pre><p>可以从 <code>icmp_seq</code> 序号看出来大约有一半的报文丢掉了，和延迟类似，丢包率也可以增加一个相关系数，表示后一个报文丢包概率和它前一个报文的相关性：</p>
<pre><code># tc qdisc change dev eth0 root netem loss 0.3% 25%
</code></pre><p>这个命令表示，丢包率是 0.3%，并且当前报文丢弃的可能性和前一个报文 25% 相关。默认的丢包模型为随机，loss 也支持 <code>state</code>（4-state Markov 模型） 和 <code>gemodel</code>（Gilbert-Elliot 丢包模型） 两种模型的丢包，因为两者都相对负责，这里也不再介绍了。</p>
<p>需要注意的是，丢包信息会发送到上层协议，如果是 TCP 协议，那么 TCP 会进行重传，所以对应用来说看不到丢包。这时候要模拟丢包，需要把 loss 配置到王桥或者路由设备上。</p>
<h3 id="3-模拟包重复"><a href="#3-模拟包重复" class="headerlink" title="3. 模拟包重复"></a>3. 模拟包重复</h3><p>报文重复和丢包的参数类似，就是重复率和相关性两个参数，比如随机产生 50% 重复的包：</p>
<pre><code>[root@node02 ~]# tc qdisc change dev enp0s8 root netem duplicate 50%
[root@node02 ~]# ping 172.17.8.100
PING 172.17.8.100 (172.17.8.100) 56(84) bytes of data.
64 bytes from 172.17.8.100: icmp_seq=1 ttl=64 time=0.705 ms
64 bytes from 172.17.8.100: icmp_seq=1 ttl=64 time=1.03 ms (DUP!)
64 bytes from 172.17.8.100: icmp_seq=2 ttl=64 time=0.710 ms
......
</code></pre><h3 id="4-模拟包损坏"><a href="#4-模拟包损坏" class="headerlink" title="4. 模拟包损坏"></a>4. 模拟包损坏</h3><p>报文损坏和报文重复的参数也类似，比如随机产生 2% 损坏的报文（在报文的随机位置造成一个比特的错误）：</p>
<pre><code># tc qdisc add dev eth0 root netem corrupt 2%
</code></pre><h3 id="5-模拟包乱序"><a href="#5-模拟包乱序" class="headerlink" title="5. 模拟包乱序"></a>5. 模拟包乱序</h3><p>网络传输并不能保证顺序，传输层 TCP 会对报文进行重组保证顺序，所以报文乱序对应用的影响比上面的几种问题要下。</p>
<p>报文乱序可前面的参数不太一样，因为上面的报文问题都是独立的，针对单个报文做操作就行，而乱序则牵涉到多个报文的重组。模拟报乱序一定会用到延迟（因为模拟乱序的本质就是把一些包延迟发送），netem 有两种方法可以做。第一种是固定的每隔一定数量的报文就乱序一次：</p>
<p>每 5 个报文（第 5、10、15…报文）会正常发送，其他的报文延迟 100ms：</p>
<pre><code># tc qdisc change dev enp0s8 root netem reorder 50% gap 3 delay 100ms
[root@node02 ~]# ping -i 0.05 172.17.8.100
PING 172.17.8.100 (172.17.8.100) 56(84) bytes of data.
64 bytes from 172.17.8.100: icmp_seq=1 ttl=64 time=0.634 ms
64 bytes from 172.17.8.100: icmp_seq=4 ttl=64 time=0.765 ms
64 bytes from 172.17.8.100: icmp_seq=2 ttl=64 time=102 ms
64 bytes from 172.17.8.100: icmp_seq=3 ttl=64 time=100 ms
64 bytes from 172.17.8.100: icmp_seq=5 ttl=64 time=100 ms
64 bytes from 172.17.8.100: icmp_seq=7 ttl=64 time=50.3 ms
64 bytes from 172.17.8.100: icmp_seq=6 ttl=64 time=100 ms
64 bytes from 172.17.8.100: icmp_seq=8 ttl=64 time=100 ms
......
</code></pre><p>要想看到 ping 报文的乱序，我们要保证发送报文的间隔小于报文的延迟时间 <code>100ms</code>，这里用 <code>-i 0.05</code> 把发送间隔设置为 <code>50ms</code>。</p>
<p>第二种方法的乱序是相对随机的，使用概率来选择乱序的报文：</p>
<pre><code>[root@node02 ~]# tc qdisc change dev enp0s8 root netem reorder 50% 15% delay 300ms
[root@node02 ~]# ping -i 0.05 172.17.8.100
PING 172.17.8.100 (172.17.8.100) 56(84) bytes of data.
64 bytes from 172.17.8.100: icmp_seq=1 ttl=64 time=0.545 ms
64 bytes from 172.17.8.100: icmp_seq=5 ttl=64 time=120 ms
64 bytes from 172.17.8.100: icmp_seq=2 ttl=64 time=300 ms
64 bytes from 172.17.8.100: icmp_seq=8 ttl=64 time=19.8 ms
64 bytes from 172.17.8.100: icmp_seq=3 ttl=64 time=301 ms
64 bytes from 172.17.8.100: icmp_seq=9 ttl=64 time=28.3 ms
64 bytes from 172.17.8.100: icmp_seq=4 ttl=64 time=300 ms
64 bytes from 172.17.8.100: icmp_seq=11 ttl=64 time=35.5 ms
......
</code></pre><p>50% 的报文会正常发送，其他报文（1-50%）延迟 300ms 发送，这里选择的延迟很大是为了能够明显看出来乱序的结果。</p>
<h2 id="两个工具"><a href="#两个工具" class="headerlink" title="两个工具"></a>两个工具</h2><p>netem 在 tc 中算是比较简单的模块，如果要实现流量控制或者精细化的过滤需要更复杂的配置。这里推荐两个小工具，它们共同的特点是用法简单，能满足特定的需求，而不用自己去倒腾 tc 的命令。</p>
<h3 id="wondershaper"><a href="#wondershaper" class="headerlink" title="wondershaper"></a>wondershaper</h3><p>netem 只能模拟网络状况，不能控制带宽，<a href="https://www.hecticgeek.com/2012/02/simple-traffic-shaping-ubuntu-linux/" target="_blank" rel="noopener">wondershaper</a> 能完美解决这个问题。wondershaper 的使用非常简单，只有三个参数：网卡名、下行限速、上行限速。比如要设置网卡下载速度为 200kb/s，上传速度为 <code>150kb/s</code>：</p>
<pre><code>wondershaper enp0s8 200 150
</code></pre><h3 id="comcast"><a href="#comcast" class="headerlink" title="comcast"></a>comcast</h3><p><a href="https://github.com/tylertreat/comcast" target="_blank" rel="noopener">comcast</a> 是一个跨平台的网络模拟工具，旨在其他平台（OSX、Windows、BSD）也提供类似网络模拟的功能。</p>
<p>它的使用也相对简单：</p>
<pre><code>$ comcast --device=eth0 --latency=250 \
    --target-bw=1000 --default-bw=1000000 \
    --packet-loss=10% \
    --target-addr=8.8.8.8,10.0.0.0/24 \
    --target-proto=tcp,udp,icmp \
    --target-port=80,22,1000:2000
</code></pre><ul>
<li><code>--device</code> 说明要控制的网卡为 <code>eth0</code></li>
<li><code>--latency</code> 指定 250ms 的延迟</li>
<li><code>--target-bw</code>指定目标带宽</li>
<li><code>--default-bw</code> 指定默认带宽</li>
<li><code>--packet-loss</code> 是丢包率</li>
<li><code>--target-addr</code>、<code>--target-proto</code>、<code>--target-port</code> 参数指定在满足这些条件的报文上实施上面的配置</li>
</ul>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>可以看出，tc 的 netem 模块主要用来模拟各种网络的异常状况，本身并没有提供宽带限制的功能，而且一旦在网卡上配置了 netem，该网卡上所有的报文都会受影响，如果想精细地控制部分报文，需要用到 tc 的 <a href="http://lartc.org/howto/lartc.qdisc.filters.html" target="_blank" rel="noopener">filter</a> 功能。</p>
<h2 id="参考资料"><a href="#参考资料" class="headerlink" title="参考资料"></a>参考资料</h2><ul>
<li><a href="https://wiki.linuxfoundation.org/networking/netem" target="_blank" rel="noopener">the Linux Foundation wiki: netem</a></li>
<li><a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.172.7072&amp;rep=rep1&amp;type=pdf" target="_blank" rel="noopener">Definition of a general and intuitive loss model for packet networks and its implementation in the Netem module in the Linux kernel</a></li>
</ul>

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        <ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#网络状况模拟"><span class="toc-text">网络状况模拟</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#1-模拟延迟传输"><span class="toc-text">1. 模拟延迟传输</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#2-模拟丢包率"><span class="toc-text">2. 模拟丢包率</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#3-模拟包重复"><span class="toc-text">3. 模拟包重复</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#4-模拟包损坏"><span class="toc-text">4. 模拟包损坏</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#5-模拟包乱序"><span class="toc-text">5. 模拟包乱序</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#两个工具"><span class="toc-text">两个工具</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#wondershaper"><span class="toc-text">wondershaper</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#comcast"><span class="toc-text">comcast</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#总结"><span class="toc-text">总结</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#参考资料"><span class="toc-text">参考资料</span></a></li></ol>
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